This invention relates to a fire-resistant rubber composition. This invention also relates to a fire-resistant hose comprising the fire-resistant rubber composition.
Rubber hose is in wide use for material transfer and fluid power applications. Fire resistance standards for hose used in transportation vehicles have become increasingly stringent. The most stringent standards apply to hose used on railway passenger trains, in particular trains that travel substantially through tunnels. Examples of such standards include British Standard BS 6853:1999 [class Ia], Norme Français or French Standard NF F 16-101 [category A1], and German Standard DIN 5510-2. Some of the requirements of these fire resistance standards are that the materials generate very low smoke density during burning, be self-extinguishing when on fire, have very high limiting oxygen index (LOI), have very high flammability temperature, only emit gasses with very low toxicity, be halogen-free and heavy-metal free, and be electrically isolating. At the same time, the hose must comply with the various conventional design requirements, such as listed in relevant railway standards including UIC 830-1 and the like. Hose performance includes such factors as burst pressure, cold flexibility, oil resistance, hot air resistance, ozone resistance, and structural integrity of various adhered layers. It is generally recognized in the art that the more fire retarding additives one mixes into the rubber in an attempt to meet fire standards, the worse the physical properties become.
Much prior art is directed to improving the fire resistance of wire and cable insulation. It is known that halogen-containing fire retardants and halogenated polymers are corrosive and generate much smoke and toxic fumes. Therefore recent prior art is directed to halogen-free compositions. For example, U.S. Pat. No. 4,675,248 discloses a blend of ethylene-vinylacetate (EVM) and hydrogenated nitrile-butadiene rubber (HNBR) with a synergistic fire-retarding combination of aluminum trihydrate (ATH) and zinc borate (ZB). In a related publication by Herman Meisenheimer, “Low smoke, non-corrosive, fire retardant cable jackets based on HNBR and EVM, Rubber World, 19 (June 1991), it is disclosed that very high values of LOI can be obtained at very high loadings of ATH, but that tear strength specifications are not met by the resulting compositions. Hose physical property requirements are different, generally more demanding, than wire and cable requirements.
U.S. Pat. No. 5,340,867 discloses a flame-retardant rubber composition based on red phosphorous and a nitrile-group containing elastomer, but the LOI is only about 27.5%. U.S. Pat. No. 4,533,687 discloses a fire-retarding synergistic blend of ATH and red phosphorous in combination with organopolysiloxane for improving the fire-resistance of olefin-type elastomers, but the highest LOI reported is is only 30%. What is needed for a hose to comply with NF F 16-101 is an LOI greater than 34%.
Various textbooks and handbooks discuss the wide array of fire retardants available for a wide array of plastic and elastomer applications. See e.g. R. G. Gann et al., “Flammability,” in H. F. Mark et al., eds., Encyclopedia of Polymer Science and Engineering, v. 7, pp. 154-210, (2d ed. 1987). However, it is clear that combinations are rarely synergistic. Sometimes, combining ingredients leads only to minor additive effects or even antagonistic effects. Known rubber compositions and hoses fall short of current and future needs. What is needed is a rubber composition with improved fire resistance and sufficient physical performance properties to meet the stringent hose performance standards discussed above.